bloody tail feathers

I have a 3 week old chick tiat I noticed had a bloody tail stump. It is one of the bigger chicks but the others are pecking at her/him. what can I do to stop this? I ahve removed her from the others. How do I fix her butt(it's not her vent.) Do I need to get some more protein in the chicks? how do I heal the bloody chick? do I wash her? I don't have a larger space for them, there are now 8 in a 50 gal rubbermaid tub.
What do i do?

Picking can be caused by more than over crowding. Is your brooder temp too high? I like to move my feeders and waterers at each feeding. A themometer would help at 3 weeks old they should be at about 85 degrees now and drop it another 5degrees each week there after. You may also try sprinkling a bit of alfalfa leaf over the litter to give them something to scratch at. A red light is more calming than a clear or white light also. And yes she maybe picking herself but the others are too it sounded like. She'll have to stay seperated till she's healed up. Good luck.